Tenant was Assaulted Outside of House

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I was hoping y'all could provide some guidance.

My tenant has gotten into some run-ins with the neighbor over the past couple of years. He states that the neighbor is a drug dealer and will often pick fights with him for no reason. My tenant will call me every time something happens. I will let him know that there is nothing I can do and he needs to contact the police department (which he now does whenever a confrontation happens). Approximately a year ago i did install security lights on the house. However because of potential liability issues, I try to stay out of the situation as much as possible. I have also advised him to get renters insurance each time this comes up and have a condition in my lease advising my tenant to get a policy. 

My tenant recently had another confrontation with the neighbor. What would you recommend for course of action? My tenant is on a month to month lease and I have let him know if he feels unsafe he is free to move. I want to make sure I limit my liability if anything ever arose out of one of these confrontations. 

Thank you all for your time. It is greatly appreciated!